MATAMOROS, MEXICO - The U.S. government is turning to elite units of Mexican marines to go after drug cartel bosses in aggressive "capture or kill" missions, providing intelligence and training to bolster what officials say is Mexico's most trustworthy and nimble force.

The effort includes more direct information-sharing and training than previously known, according to diplomats and law enforcement officials, and reflects a sense of urgency on the part of the U.S. government to find a professional partner to combat drug violence in Mexico that is seen as posing a threat to American security.

The U.S. government has long been wary of corruption among Mexican police and frustrated by the slow response of the Mexican army. The decision to rely on the marines has enabled that force to carry out the kind of rapid-strike operations undertaken by U.S. forces against Taliban leaders Afghanistan.

Based in the U.S. Embassy and in consulates in conflict zones along the border such as Matamoros, agents with the Drug Enforcement Administration deliver "intelligence packages" about the location of drug bosses to the Mexican marines, who then charge into action, often within hours, sometimes capturing, sometimes killing their quarry in spectacular urban firefights.

Mexican officials deny that the U.S. military is training Mexican marines, and the Pentagon declines to discuss the training, but U.S. officials confirm in interviews and recently leaked diplomatic cables that the U.S. military is conducting urban-combat and counterinsurgency instruction in Mexico and the United States.

Employing the marines is a strategy not without risks - the Mexican navy, like the army, lacks real transparency, is not subject to broad civilian oversight and has embarrassed itself. U.S. officials were shocked to see the bloodied, half-naked corpse of cartel leader Arturo Beltran Leyva covered in peso bank notes and jewelry in "trophy photographs" splashed on the front pages of newspapers hours after his death.

Human rights groups say complaints against the marines may increase as their role in the drug war expands.

"By bringing the navy in for short-term operations - to take somebody out and leave again - they preserve some level of immunity," said Maureen Meyer, a Mexico expert at the Washington Office on Latin America. "But having the navy doing patrols on the streets is blurring the lines, and that's the big risk now."

In a cable dated Jan. 29, John Feeley, the deputy chief of mission for the U.S. Embassy in Mexico City, praised the Mexican marines and said that "our ties with the military have never been closer in terms of not only equipment transfers and training, but also the kinds of intelligence exchanges that are essential to making inroads against organized crime." The cable was written before a high-level Defense Bilateral Working Group meeting in Mexico in February.

The diplomat wrote that "for the first time," the Mexican army is "following the Navy's lead" and "has asked for SOF training." SOF is an acronym for Special Operations Force. The cables were provided by the anti-secrecy group WikiLeaks to media sites Thursday night.

In another cable, after a December 2009 operation that resulted in Beltran Leyva's killing in a luxury condo in Cuernavaca, U.S. Ambassador Carlos Pascual also praised the marines corps for "its emerging role as the key player in the counter-narcotics fight."

According to the ambassador, the marine unit that led the operation had been "extensively trained" by the U.S. Northern Command, the Pentagon's joint operations center in Colorado that oversees and coordinates defense in North America, including Mexico.